Network Working Group L. Eggert
Request for Comments: 5482 Nokia
Category: Standards Track F. Gont
UTN/FRH
March 2009
TCP User Timeout Option
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
The TCP user timeout controls how long transmitted data may remain
unacknowledged before a connection is forcefully closed. It is a
local, per-connection parameter. This document specifies a new TCP
option -- the TCP User Timeout Option -- that allows one end of a TCP
connection to advertise its current user timeout value. This
information provides advice to the other end of the TCP connection to
adapt its user timeout accordingly. Increasing the user timeouts on
both ends of a TCP connection allows it to survive extended periods
without end-to-end connectivity. Decreasing the user timeouts allows
busy servers to explicitly notify their clients that they will
maintain the connection state only for a short time without
connectivity.
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RFC 5482 TCP User Timeout Option March 2009
Table of Contents
1. Introduction ....................................................22. Conventions .....................................................33. Operation .......................................................43.1. Changing the Local User Timeout ............................53.2. UTO Option Reliability .....................................83.3. Option Format ..............................................83.4. Reserved Option Values .....................................94. Interoperability Issues .........................................94.1. Middleboxes ................................................94.2. TCP Keep-Alives ...........................................105. Programming and Manageability Considerations ...................106. Security Considerations ........................................107. IANA Considerations ............................................128. Acknowledgments ................................................129. References .....................................................129.1. Normative References ......................................129.2. Informative References ....................................131. Introduction
The Transmission Control Protocol (TCP) specification [RFC0793]
defines a local, per-connection "user timeout" parameter that
specifies the maximum amount of time that transmitted data may remain
unacknowledged before TCP will forcefully close the corresponding
connection. Applications can set and change this parameter with OPEN
and SEND calls. If an end-to-end connectivity disruption lasts
longer than the user timeout, a sender will receive no
acknowledgments for any transmission attempt, including keep-alives,
and it will close the TCP connection when the user timeout occurs.
This document specifies a new TCP option -- the TCP User Timeout
Option (UTO) -- that allows one end of a TCP connection to advertise
its current user timeout value. This information provides advice to
the other end of the connection to adapt its user timeout
accordingly. That is, TCP remains free to disregard the advice
provided by the UTO option if local policies suggest it to be
appropriate.
Increasing the user timeouts on both ends of a TCP connection allows
it to survive extended periods without end-to-end connectivity.
Decreasing the user timeouts allows busy servers to explicitly notify
their clients that they will maintain the connection state only for a
short time without connectivity.
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RFC 5482 TCP User Timeout Option March 2009
In the absence of an application-specified user timeout, the TCP
specification [RFC0793] defines a default user timeout of 5 minutes.
The Host Requirements RFC [RFC1122] refines this definition by
introducing two thresholds, R1 and R2 (R2 > R1), that control the
number of retransmission attempts for a single segment. It suggests
that TCP should notify applications when R1 is reached for a segment,
and close the connection when R2 is reached. [RFC1122] also defines
the recommended values for R1 (3 retransmissions) and R2 (100
seconds), noting that R2 for SYN segments should be at least 3
minutes. Instead of a single user timeout, some TCP implementations
offer finer-grained policies. For example, Solaris supports
different timeouts depending on whether a TCP connection is in the
SYN-SENT, SYN-RECEIVED, or ESTABLISHED state [SOLARIS].
Although some TCP implementations allow applications to set their
local user timeout, TCP has no in-protocol mechanism to signal
changes to the local user timeout to the other end of a connection.
This causes local changes to be ineffective in allowing a connection
to survive extended periods without connectivity, because the other
end will still close the connection after its user timeout expires.
The ability to inform the other end of a connection about the local
user timeout can improve TCP operation in scenarios that are
currently not well supported. One example of such a scenario is
mobile hosts that change network attachment points. Such hosts,
maybe using Mobile IP [RFC3344], HIP [RFC4423], or transport-layer
mobility mechanisms [TCP_MOB], are only intermittently connected to
the Internet. In between connected periods, mobile hosts may
experience periods without end-to-end connectivity. Other factors
that can cause transient connectivity disruptions are high levels of
congestion or link or routing failures inside the network. In these
scenarios, a host may not know exactly when or for how long
connectivity disruptions will occur, but it might be able to
determine an increased likelihood for such events based on past
mobility patterns and thus benefit from using longer user timeouts.
In other scenarios, the time and duration of a connectivity
disruption may even be predictable. For example, a node in space
might experience connectivity disruptions due to line-of-sight
blocking by planetary bodies. The timing of these events may be
computable from orbital mechanics.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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RFC 5482 TCP User Timeout Option March 20093. Operation
Use of the TCP User Timeout Option can be either enabled on a per-
connection basis, e.g., through an API option, or controlled by a
system-wide setting. TCP maintains four per-connection state
variables to control the operation of the UTO option, three of which
(ADV_UTO, ENABLED, and CHANGEABLE) are new:
USER_TIMEOUT
TCP's USER TIMEOUT parameter, as specified in [RFC0793].
ADV_UTO
UTO option advertised to the remote TCP peer. This is an
application-specified value, and may be specified on a system-wide
basis. If unspecified, it defaults to the default system-wide
USER TIMEOUT.
ENABLED (Boolean)
Flag that controls whether the UTO option is enabled for a
connection. This flag applies to both sending and receiving.
Defaults to false.
CHANGEABLE (Boolean)
Flag that controls whether USER_TIMEOUT (TCP's USER TIMEOUT
parameter) may be changed based on an UTO option received from the
other end of the connection. Defaults to true and becomes false
when an application explicitly sets USER_TIMEOUT.
Note that an exchange of UTO options between both ends of a
connection is not a binding negotiation. Transmission of a UTO
option is a suggestion that the other end consider adapting its user
timeout. This adaptation only happens if the other end of the
connection has explicitly allowed it (both ENABLED and CHANGEABLE are
true).
Before opening a connection, an application that wishes to use the
UTO option enables its use by setting ENABLED to true. It may choose
an appropriate local UTO by explicitly setting ADV_UTO; otherwise,
UTO is set to the default USER TIMEOUT value. Finally, the
application should determine whether it will allow the local USER
TIMEOUT to change based on received UTO options from the other end of
a connection. The default is to allow this for connections that do
not have specific user timeout concerns. If an application
explicitly sets the USER_TIMEOUT, CHANGEABLE MUST become false in
order to prevent UTO options (from the other end) from overriding
local application requests. Alternatively, applications can set or
clear CHANGEABLE directly through API calls.
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RFC 5482 TCP User Timeout Option March 2009
Performing these steps before an active or passive open causes UTO
options to be exchanged in the SYN and SYN-ACK packets and is a
reliable way to initially exchange, and potentially adapt to, UTO
values. TCP implementations MAY provide system-wide default settings
for the ENABLED, ADV_UTO and CHANGEABLE connection parameters.
In addition to exchanging UTO options in the SYN segments, a
connection that has enabled UTO options SHOULD include a UTO option
in the first packet that does not have the SYN flag set. This helps
to minimize the amount of state information TCP must keep for
connections in non-synchronized states. Also, it is particularly
useful when mechanisms such as "SYN cookies" [RFC4987] are
implemented, allowing a newly-established TCP connection to benefit
from the information advertised by the UTO option, even if the UTO
contained in the initial SYN segment was not recorded.
A host that supports the UTO option SHOULD include one in the next
possible outgoing segment whenever it starts using a new user timeout
for the connection. This allows the other end of the connection to
adapt its local user timeout accordingly. A TCP implementation that
does not support the UTO option MUST silently ignore it [RFC1122],
thus ensuring interoperability.
Hosts MUST impose upper and lower limits on the user timeouts they
use for a connection. Section 3.1 discusses user timeout limits and
potentially problematic effects of some user timeout settings.
Finally, it is worth noting that TCP's option space is limited to 40
bytes. As a result, if other TCP options are in use, they may
already consume all the available TCP option space, thus preventing
the use of the UTO option specified in this document. Therefore, TCP
option space issues should be considered before enabling the UTO
option.
3.1. Changing the Local User Timeout
When a host receives a TCP User Timeout Option, it must decide
whether to change the local user timeout of the corresponding
connection. If the CHANGEABLE flag is false, USER_TIMEOUT MUST NOT
be changed, regardless of the received UTO option. Without this
restriction, the UTO option would modify TCP semantics, because an
application-requested USER TIMEOUT could be overridden by peer
requests. In this case TCP SHOULD, however, notify the application
about the user timeout value received from the other end system.
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RFC 5482 TCP User Timeout Option March 2009
In general, unless the application on the local host has requested a
specific USER TIMEOUT for the connection, CHANGEABLE will be true and
hosts SHOULD adjust the local TCP USER TIMEOUT (USER_TIMEOUT) in
response to receiving a UTO option, as described in the remainder of
this section.
The UTO option specifies the user timeout in seconds or minutes,
rather than in number of retransmissions or round-trip times (RTTs).
Thus, the UTO option allows hosts to exchange user timeout values
from 1 second to over 9 hours at a granularity of seconds, and from 1
minute to over 22 days at a granularity of minutes.
Very short USER TIMEOUT values can affect TCP transmissions over
high-delay paths. If the user timeout occurs before an
acknowledgment for an outstanding segment arrives, possibly due to
packet loss, the connection closes. Many TCP implementations default
to USER TIMEOUT values of a few minutes. Although the UTO option
allows suggestion of short timeouts, applications advertising them
should consider these effects.
Long USER TIMEOUT values allow hosts to tolerate extended periods
without end-to-end connectivity. However, they also require hosts to
maintain the TCP state information associated with connections for
long periods of time. Section 6 discusses the security implications
of long timeout values.
To protect against these effects, implementations MUST impose limits
on the user timeout values they accept and use. The remainder of
this section describes a RECOMMENDED scheme to limit TCP's USER
TIMEOUT based on upper and lower limits.
Under the RECOMMENDED scheme, and when CHANGEABLE is true, each end
SHOULD compute the local USER TIMEOUT for a connection according to
this formula:
USER_TIMEOUT = min(U_LIMIT, max(ADV_UTO, REMOTE_UTO, L_LIMIT))
Each field is to be interpreted as follows:
USER_TIMEOUT
USER TIMEOUT value to be adopted by the local TCP for this
connection.
U_LIMIT
Current upper limit imposed on the user timeout of a connection by
the local host.
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RFC 5482 TCP User Timeout Option March 2009
ADV_UTO
User timeout advertised to the remote TCP peer in a TCP User
Timeout Option.
REMOTE_UTO
Last user timeout value received from the other end in a TCP User
Timeout Option.
L_LIMIT
Current lower limit imposed on the user timeout of a connection by
the local host.
The RECOMMENDED formula results in the maximum of the two advertised
values, adjusted for the configured upper and lower limits, to be
adopted for the user timeout of the connection on both ends. The
rationale is that choosing the maximum of the two values will let the
connection survive longer periods without end-to-end connectivity.
If the end that announced the lower of the two user timeout values
did so in order to reduce the amount of TCP state information that
must be kept on the host, it can close or abort the connection
whenever it wants.
It must be noted that the two endpoints of the connection will not
necessarily adopt the same user timeout.
Enforcing a lower limit (L_LIMIT) prevents connections from closing
due to transient network conditions, including temporary congestion,
mobility hand-offs, and routing instabilities.
An upper limit (U_LIMIT) can reduce the effect of resource exhaustion
attacks. Section 6 discusses the details of these attacks.
Note that these limits MAY be specified as system-wide constants or
at other granularities, such as on per-host, per-user, per-outgoing-
interface, or even per-connection basis. Furthermore, these limits
need not be static. For example, they MAY be a function of system
resource utilization or attack status and could be dynamically
adapted.
The Host Requirements RFC [RFC1122] does not impose any limits on the
length of the user timeout. However, it recommends a time interval
of at least 100 seconds. Consequently, the lower limit (L_LIMIT)
SHOULD be set to at least 100 seconds when following the RECOMMENDED
scheme described in this section. Adopting a user timeout smaller
than the current retransmission timeout (RTO) for the connection
would likely cause the connection to be aborted unnecessarily.
Therefore, the lower limit (L_LIMIT) MUST be larger than the current
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RFC 5482 TCP User Timeout Option March 2009
retransmission timeout (RTO) for the connection. It is worth noting
that an upper limit may be imposed on the RTO, provided it is at
least 60 seconds [RFC2988].
3.2. UTO Option Reliability
The TCP User Timeout Option is an advisory TCP option that does not
change processing of subsequent segments. Unlike other TCP options,
it need not be exchanged reliably. Consequently, the specification
does not define a reliability handshake for UTO option exchanges.
When a segment that carries a UTO option is lost, the other end will
simply not have the opportunity to update its local USER TIMEOUT.
Implementations MAY implement local mechanisms to improve delivery
reliability, such as retransmitting a UTO option when they retransmit
a segment that originally carried it, or "attaching" the option to a
byte in the stream and retransmitting the option whenever that byte
or its ACK are retransmitted.
It is important to note that although these mechanisms can improve
transmission reliability for the UTO option, they do not guarantee
delivery (a three-way handshake would be required for this).
Consequently, implementations MUST NOT assume that UTO options are
transmitted reliably.
3.3. Option Format
Sending a TCP User Timeout Option informs the other end of the
connection of the current local user timeout and suggests that the
other end adapt its user timeout accordingly. The user timeout value
included in a UTO option contains the ADV_UTO value that is expected
to be adopted for the TCP's USER TIMEOUT parameter during the
synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-
WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK). Connections in other
states MUST use the default timeout values defined in [RFC0793] and
[RFC1122].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind = 28 | Length = 4 |G| User Timeout |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(One tick mark represents one bit.)
Figure 1: Format of the TCP User Timeout Option
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RFC 5482 TCP User Timeout Option March 2009
Figure 1 shows the format of the TCP User Timeout Option. It
contains these fields:
Kind (8 bits)
This MUST be 28, i.e., the TCP option number [RFC0793] that has
been assigned by IANA (see Section 7).
Length (8 bits)
Length of the TCP option in octets [RFC0793]; its value MUST be 4.
Granularity (1 bit)
Granularity bit, indicating the granularity of the "User Timeout"
field. When set (G = 1), the time interval in the "User Timeout"
field MUST be interpreted as minutes. Otherwise (G = 0), the time
interval in the "User Timeout" field MUST be interpreted as
seconds.
User Timeout (15 bits)
Specifies the user timeout suggestion for this connection. It
MUST be interpreted as a 15-bit unsigned integer. The granularity
of the timeout (minutes or seconds) depends on the "G" field.
3.4. Reserved Option Values
A TCP User Timeout Option with a "User Timeout" field of zero and a
"Granularity" bit of either minutes (1) or seconds (0) is reserved
for future use. Current TCP implementations MUST NOT send it and
MUST ignore it upon reception.
4. Interoperability Issues
This section discusses interoperability issues related to introducing
the TCP User Timeout Option.
4.1. Middleboxes
A TCP implementation that does not support the TCP User Timeout
Option MUST silently ignore it [RFC1122], thus ensuring
interoperability. In a study of the effects of middleboxes on
transport protocols, Medina et al. have shown that the vast majority
of modern TCP stacks correctly handle unknown TCP options [MEDINA].
In this study, 3% of connections failed when an unknown TCP option
appeared in the middle of a connection. Because the number of
failures caused by unknown options is small and they are a result of
incorrectly implemented TCP stacks that violate existing requirements
to ignore unknown options, they do not warrant special measures.
Thus, this document does not define a mechanism to negotiate support
of the TCP User Timeout Option during the three-way handshake.
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RFC 5482 TCP User Timeout Option March 2009
Implementations may want to exchange UTO options on the very first
data segments after the three-way handshake to determine if such a
middlebox exists on the path. When segments carrying UTO options are
persistently lost, an implementation should turn off the use of UTO
for the connection. When the connection itself is reset, an
implementation may be able to transparently re-establish another
connection instance that does not use UTO before any application data
has been successfully exchanged.
Stateful firewalls usually time out connection state after a period
of inactivity. If such a firewall exists along the path, it may
close or abort connections regardless of the use of the TCP User
Timeout Option. In the future, such firewalls may learn to parse the
TCP User Timeout Option in unencrypted TCP segments and adapt
connection state management accordingly.
4.2. TCP Keep-Alives
Some TCP implementations, such as those in BSD systems, use a
different abort policy for TCP keep-alives than for user data. Thus,
the TCP keep-alive mechanism might abort a connection that would
otherwise have survived the transient period without connectivity.
Therefore, if a connection that enables keep-alives is also using the
TCP User Timeout Option, then the keep-alive timer MUST be set to a
value larger than that of the adopted USER TIMEOUT.
5. Programming and Manageability Considerations
The IETF specification for TCP [RFC0793] includes a simple, abstract
application programming interface (API). Similarly, the API for the
UTO extension in Section 3 is kept abstract. TCP implementations,
however, usually provide more complex and feature-rich APIs. The
"socket" API that originated with BSD Unix and is now standardized by
POSIX is one such example [POSIX]. It is expected that TCP
implementations that choose to include the UTO extension will extend
their API to allow applications to use and configure its parameters.
The MIB objects defined in [RFC4022] and [RFC4898] allow management
of TCP connections. It is expected that revisions to these documents
will include definitions of objects for managing the UTO extension
defined in this document.
6. Security Considerations
Lengthening user timeouts has obvious security implications.
Flooding attacks cause denial of service by forcing servers to commit
resources for maintaining the state of throw-away connections.
However, TCP implementations do not become more vulnerable to simple
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RFC 5482 TCP User Timeout Option March 2009
SYN flooding by implementing the TCP User Timeout Option, because
user timeouts exchanged during the handshake only affect the
synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,
CLOSING, LAST-ACK), which simple SYN floods never reach.
However, when an attacker completes the three-way handshakes of its
throw-away connections, it can amplify the effects of resource
exhaustion attacks because the attacked server must maintain the
connection state associated with the throw-away connections for
longer durations. Because connection state is kept longer, lower-
frequency attack traffic, which may be more difficult to detect, can
already exacerbate resource exhaustion.
Several approaches can help mitigate this issue. First,
implementations can require prior peer authentication, e.g., using
IPsec [RFC4301] or TCP-MD5 [RFC2385], before accepting long user
timeouts for the peer's connections. (Implementors that decide to
use TCP-MD5 for this purpose are encouraged to monitor the
development of TCP-AO [AUTH_OPT], its designated successor, and
update their implementation when it is published as an RFC.) A
similar approach is for a host to start accepting long user timeouts
for an established connection only after in-band authentication has
occurred, for example, after a TLS handshake across the connection
has succeeded [RFC5246]. Although these are arguably the most
complete solutions, they depend on external mechanisms to establish a
trust relationship.
A second alternative that does not depend on external mechanisms
would introduce a per-peer limit on the number of connections that
may use increased user timeouts. Several variants of this approach
are possible, such as fixed limits or shortening accepted user
timeouts with a rising number of connections. Although this
alternative does not eliminate resource exhaustion attacks from a
single peer, it can limit their effects. Reducing the number of
high-UTO connections a server supports in the face of an attack turns
that attack into a denial-of-service attack against the service of
high-UTO connections.
Per-peer limits cannot protect against distributed denial-of-service
attacks, where multiple clients coordinate a resource exhaustion
attack that uses long user timeouts. To protect against such
attacks, TCP implementations could reduce the duration of accepted
user timeouts with increasing resource utilization.
TCP implementations under attack may be forced to shed load by
resetting established connections. Some load-shedding heuristics,
such as resetting connections with long idle times first, can
negatively affect service for intermittently connected, trusted peers
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RFC 5482 TCP User Timeout Option March 2009
that have suggested long user timeouts. On the other hand, resetting
connections to untrusted peers that use long user timeouts may be
effective. In general, using the peers' level of trust as a
parameter during the load-shedding decision process may be useful.
Note that if TCP needs to close or abort connections with a long TCP
User Timeout Option to shed load, these connections are still no
worse off than without the option.
Finally, upper and lower limits on user timeouts, discussed in
Section 3.1, can be an effective tool to limit the impact of these
sorts of attacks.
7. IANA Considerations
This section is to be interpreted according to [RFC5226].
This document does not define any new namespaces. IANA has allocated
a new 8-bit TCP option number (28) for the UTO option from the "TCP
Option Kind Numbers" registry maintained at http://www.iana.org.
8. Acknowledgments
The following people have improved this document through thoughtful
suggestions: Mark Allman, Caitlin Bestler, David Borman, Bob Braden,
Scott Brim, Marcus Brunner, Wesley Eddy, Gorry Fairhurst, Abolade
Gbadegesin, Ted Faber, Guillermo Gont, Tom Henderson, Joseph Ishac,
Jeremy Harris, Alfred Hoenes, Phil Karn, Michael Kerrisk, Dan Krejsa,
Jamshid Mahdavi, Kostas Pentikousis, Juergen Quittek, Anantha
Ramaiah, Joe Touch, Stefan Schmid, Simon Schuetz, Tim Shepard, and
Martin Stiemerling.
Lars Eggert is partly funded by [TRILOGY], a research project
supported by the European Commission under its Seventh Framework
Program.
Fernando Gont wishes to thank Secretaria de Extension Universitaria
at Universidad Tecnologica Nacional and Universidad Tecnologica
Nacional/Facultad Regional Haedo for supporting him in this work.
9. References9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
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